Method of converting metal powder into bar stock

ABSTRACT

A METHOD AND APPARATUS FOR PRODUCING ROD-SHAPED METAL COMPACTS IN WHICH THE POWDER CHARGE IS FED UNDER VACUUM INTO A CONTINUOUSLY FORMED JACKETING TUBE. THE JACKETING CORE IN THEN HOT ROLLED TO FORM THE SINTER COM-   PACT; THE AXIAL PULL OF THE ROLLING OPERATION PREVENTING THE BULGING OF TUBE JACKET. THE JACKET IS THEN REMOVED CHEMICALLY OR BY MACHINING.

1973 HUNG-CHI CHAO ,708, 84

METHOD OF CONVERTING METAL POWDER INTO BAR STOCK Filed Jan. 26, 1970 INVENTOR HUNG "CHI CH4 0 Attorney United States Patent Office 3,708,284 Patented Jan. 2, 1973 3,708,284 METHOD OF CONVERTING METAL POWDER INTO BAR STOCK Hung-Chi Chan, Monroeville Borough, Pa., assignor to United States Steel Corporation Filed Jan. 26, 1970, Ser. No. 5,463 Int. Cl. B221? 1/00 U.S. Cl. 75-208 CS 8 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for producing rod-shaped metal compacts in which the powder charge is fed under vacuum into a continuously formed jacketing tube. The jacketed core is then hot rolled to form the sinter compact; the axial pull of the rolling operation preventing the bulging of tube jacket. The jacket is then removed chemically or by machining.

This invention relates to an improved process for producing rod or bar metal shapes by hot rolling metal powders packed in tubular containers under vacuum. Powder metallurgy techniques are employed to produce metal articles having a variety of shapes which are difiicult to produce by more conventional processes such as rolling, forging, or continuous casting techniques. Additionally, these techniques are used to achieve special purposes that cannot be realized by other processes. Such special purposes include the dispersion of different phases; the production of porous materials and the production of highly refractory (high melting point) metals.

Since metal powder costs are several times that of ingot metals and since expensive equipment and high pressures are generally required, commercial practice has been generally limited to the production of relatively small metal parts. Cylindrical compacts of the order of a few feet in length have been obtained by procedures such as those described in U.S. Pat. No. 3,340,055, in which a powder packed container is evacuated, sealed and heated to elevated temperatures and then compacted in suitable dies. Subsequent to this compacting, the articles may then be further processed in conventional rolling facilities. However, continuous, low cost production of articles from metal powders has not been achieved by the prior art.

It is therefore an object of this invention to produce powder compacts of unlimited lengths, while eliminating the intermediate and separate compacting operation.

It is a further object of this invention to provide a continuous process which may be used in conjunction with conventional type tube making processes and thus greatly reduce costs.

It is another object of this invention to provide a process and apparatus for producing high length to diameter shapes, in which material yields of greater than 99% may be attained.

These and further objects of the invention will become more apparent by reference to the following disclosure and the figure which illustrates a preferred apparatus for carrying out the process of this invention.

With reference to the figure, the charge of metal powder or chips, of any suitable size (which will generally be less than A; the size of the smallest dimension of the cross-section of the container, and preferably of spheroidal-like shape to achieve high loose packing density) is loaded into a feeder bin 1 which is subsequently evacuated. After evacuation, gate valve 2 is opened to permit the powder to enter a vertical gravitational feeder 3, the function of which may be further aided by a suitable vibrator 4. The feeder is provided with terminal nozzle 5 located in a container tube 6 at a suitable distance below the electrodes of seam welder 7. Container tube 6 is continuously produced from suitable metal strip 8 by suitable seamed-tubing process and equipment. Welded container tube 6, with loose packed powder in its confines, moves downward, driven by guiding roll 9. The feeder 3, tube-forming rolls 10, welding electrodes 7, pressure rolls 11, and guiding and forming rolls 9 and 10, respectively, are maintained under high vacuum in enclosed chamber 12. In order to maintain the required high vacuum level in chamber 12 it is generally necessary to employ intermediary vacuum chambers 13a and 13b which are located, respectively, just prior to the point where the metal strip enters the main vacuum chamber and just subsequent to the point where the welded tube leaves the main vacuum chamber. Inert gas seals 14, at positive pressure, are employed at the entrance and exit points of rough chambers 13a and 13b. After the powder packed container tube leaves the rough vacuum chamber 13b, it is bent to a horizontal position by a sequence of guide rolls 15a and 15b (which may also perform variable amounts of cold reduction) after which it is heated in furnace 16 to hot rolling temperature and then hot rolled to the desired reduction in rolls 17. The uniform radial squeeze associated with the axial pulling operations serves to compact the powder while preventing bulging of the container tube 5 into the compacted article. Material conversions of greater than 99% can be obtained, since only a portion of the top and tail ends of the clad continuous compact are chopped off, which portions are small when compared with the large product lengths which may be realized by this process. Products approaching full density may be achieved. The term full density, as used herein, refers to the density which is attainable using metal powders of a particular size, considering that inclusions are of considerably lower density than that of the metal itself. This density will generally be lower than the density of the pure metal.

It will be obvious to those skilled in the art that a number of apparatus variations may be employed without departing from the spirit of the invention. While the container tube is depicted as cylindrical, various cross-sectional enclosure configurations such as squares and rectangles may be employed. Horizontally aligned formingrolls may be employed, in which case the powder feed would be normal to and above the plane of travel of the sheet strip, instead of the co-current feeding described in the figure. Instead of the simple gravity feed system, more intricate feed systems such as plungers or even magnetic attraction (for ferromagnetic powders) may be employed in which case there would also be no necessity for vertically aligned forming rolls. Any suitable method of seam welding may be employed, e.g. resistance welding, arc welding or torch welding. A butt-welding system may be provided in which the whole of the main vacuum chamber is heated or in which just the skelp-container tube is heated (as by induction heating) so that tube ends may be welded, by simply being firmly pressed together.

A further feature associated with the continuous process of the invention is that several different alloy powders can be continuously hot rolled in a single tubular container by sequential powder feeding of the powders. A spacer metal powder of the same material as the container may be employed, if desired, so that it can be chemically dissolved in the same operation as the jacket.

The pressures and temperatures to be employed depends on the composition of the metal powders (prealloyed or a mixture of component metals) as does the jacket material. The jacket material and thickness are selected so that the jacket 1 will protect the powder during hot rolling by maintaining the vacuum, 2 will not alloy with the powder, and 3 can easily and economically be removed by stripping, machining or chemical dissolution.

The following specific example is provided to further illustrate the concept of the invention. Low carbon sheet was employed as the container tube material. Stainless steel powder (type 304) was fed into the container tube and packed (utilizing vibration) under a vacuum of 10* mm. Hg, to a loose packed density of almost 64%. The experimental tube was then capped at the tail end and hot rolled (at 2250 F.) to provide a total reduction of about 70%. The low carbon steel jacket was dissolved and the resulting compact was smooth in surface and of full density. Of the 118 inch product obtained, only 1% inches needed to be cropped ofi at each of the ends, thereby providing a yield of about 97.5%

I claim:

1. A method of producing powder metal compacts having a high length to cross-section ratio, comprising (a) in a zone maintained at a pressure substantially less than atmospheric, continuously forming a sheet of jacketing metal into a tubular configuration by welding the edges thereof to provide an elongated, hollow tube,

(b) within said zone, continuously feeding a charge of metal powder into the so-formed tube at a rate and in a manner so as to cause said charge to be packed to a high loose packed density as a core within said tube, thereby providing an elongated jacketed core, and

(c) heating said jacketed core to hot rolling temperature and thereafter reducing the cross-section thereof by rolling to an extent sufficient to provide a core approaching full metal density, wherein the speed of the rolling operation is matched with the speed of the welding operation so as to provide a continuous axial pull sufficient to prevent bulging of the jacket.

2. The method of claim 1, wherein prior to said heating of step c, said jacketed core is cold-rolled to reduce the cross-section thereof and thereby provide a densified core.

3. The method of claim 2 in which said metal powder charge is selected from the group consisting of pure metal powders, alloy metal powders and mixtures thereof.

4. The method of claim 3 which the welding of the formed sheet edges is accomplished by electric resistance welding.

5. The method of claim 4 in which the metal powder charge is vibrated to form a high loose packing density.

4 6. The method of claim 5 wherein subsequent to the hot rolling operation the tube jacket is removed from the core compact.

7. An apparatus for producing metal powder compacts 5 of high length to cross-section ratio which includes:

(a) means for feeding and conveying a sheet of metal;

(b) means for welding the lateral edges of said sheet of metal to form a jacketing tube;

(c) means for feeding a metal powder charge into the so-formed jacketing tube;

(d) means for packing said powder charge to form a high loose packed density core within the jacketing tube, the means of (b), (c) and (d) being maintained within an enclosure (e) means for maintaining a vacuum within said enclosure (f) furnace means for heating the resulting jacketed core;

(g) rolling means, subsequent to said furnace means for both exerting an axial pull on and reducing the cross-section of said jacketed core to thereby form a densified metal compact.

8. The apparatus of claim 7, wherein the means of (b) consists of an electrical resistance welder and the means of (d) are vibrating means.

References Cited UNITED STATES PATENTS 11/1964 Andersen et al. 75-214 3/1966 Unrath 2933 2/1965 Takahshi et a1. 75214 6/1966 Kazakov 72--219 OTHER REFERENCES CARL D. QUARFORTH, Primary Examiner R. E. SCHAFER, Assistant Examiner US. Cl. X.R.

189; 2933 D, 33 T; 752l4, 226 

